The analysis of randomly occurring signals received in an environment containing large numbers of pulsed and continuous wave signals, particularly where many of these signals can occur simultaneously, and where no signals should be missed, would normally require a great deal of complex and expensive electronic equipment. For example, if pulsed signals of 0.1 microsecond duration are not to be missed and if these signals can occur anywhere in frequency over a 2 GHZ wide band, with the equivalent of 16 bits of total data required from each signal, the data processing rate can run as high as 10 billion bits per second. Equipment capable of processing data at such rates would be prohibitively expensive for many applications.
When several signal parameters such as frequency, angle of arrival, signal strength, pulse width, etc. are all to be determined from a very large number of randomly arriving signals that should be intercepted with nearly a 100% probability, several problems emerge. How is simultaneous arrival of several signals together handled? How do all the parameters for a single signal get collected for a signal that might exist for a microsecond? How can one measurement be kept from interfering from the others that are simultaneously occurring? And how can such complex analysis be handled with a reasonable amount of complexity?
Accepted approaches that attempt to provide such capability include frequency swept methods, which have probability of intercept problems, and channelized filters which become prohibitively large and expensive. High resolution, high probability of intercept angle of arrival measurements with reasonably sized antennas are only possible with some form of phase comparative measurement, yet practical phase comparative angle of arrival measurements impose difficult phase tracking requirements, particularly as a function of signal level fluctuation and also produce inherent ambiguities. The ability to use the same equipment for frequency and angle of arrival measurements is advantageous. Time sharing of equipment is similarly advantageous. When simultaneously arriving signals are bunched closely in frequency and level, mutual interference will occur negating such measurements. A receiver must respond intelligently to such conditions.
Although frequency is an important characteristic to be measured, in most cases it is not the absolute frequency accuracy but the frequency resolution or discrimination that is important. Also angle of arrival data from each signal becomes more useful as its precision and resolution is increased.